1. Field of the Invention
The present invention relates to hydromechanical fuel pump systems for supplying timing fluid and fuel to high pressure fuel injectors. In particular, to such a system for supplying the timing fluid and fuel to an injector at a controlled pressure which may be adjusted in accordance with engine operating conditions.
2. Description of Related Art
In U.S. Pat. No. 4,721,247, issued to one of the present co-inventors, a high pressure unit fuel injector is disclosed which is designed to inject precisely metered quantities of fuel at a timing that is controllable as a function of the amount of timing fluid supplied to a variable timing fluid chamber. In such an injector, the amount of timing fluid and the amount of fuel to be injected are a function of the pressure of the fuel supplied to the injection chamber and used as a timing fluid in the timing chamber. If only pressure affects the quantity metered, the system is "P" metered. If the time period during which fuel is supplied also affects the quantity metered, the Such injectors are known as "PT" injectors. Other examples of such unit fuel injectors are identified in the Background Art portion of U.S. Pat. No. 4,721,247, as well.
As can be appreciated, the effectiveness of high pressure fuel injectors of the "P" or "PT" type is dependent on the effectiveness of the fuel supply system used for supplying the timing fluid and the fuel to be injected. In FIG. 3 of U.S. Pat. No. 4,721,247, an electronically controlled fuel supply system for such fuel injectors is diagrammatically depicted. This system utilizes an electronic control unit for monitoring throttle position and the output of sensors measuring such factors as engine temperature and the like to operate an electronically controlled fuel supply valve arrangement that regulates the supplying of fuel to supply rails associated with a plurality of injectors of an engine and also controls the pressure of the fluid in the timing rail that supplies timing fluid to the timing chambers of the injectors. However, electronic controls are expensive, require expensive equipment to service and may require m ore service than an equivalent hydromechanical control.
Hydromechanical controls for fuel injection systems, including those of the "PT" type, are known. For example, in FIG. 1, a prior art "PT" governor is illustrated which may be used to control presure and thereby the quantity of fuel supplied to fuel injectors as a function of engine rpm in its function as a governor for setting the idle speed and maximum speed of operation of a fuel injected engine with which it is associated. In particular, this known governor utilizes a flyweight arrangement consisting of governor weights that are pivotally carried by a weight carrier that is spring biased by weight assist and torque springs. The weight carrier is caused to rotate at a rpm corresponding to that of the engine so that as engine speed increases, the rotational speed of the weight carrier increases. As a result, the governor weights pivot under the effects of centrifugal force and thereby cause axial displacement of a shaft received within a governor sleeve of a governor barrel This axial displacement controls flow between a supply port, by which fuel is received by the governor, and idle, fuel out, and bypass ports. Flow from the shaft to the bypass port is controlled by a pressure control button that is acted upon by an idle spring and which is received within a button guide that is biased by a governor spring.
By balancing the force supplied by the idle spring on the pressure control button relative to the biased flyweight force applied at the desired low idle speed, the engine idle speed can be controlled. Similarly, by balancing the force of the governor spring against the biased flyweight force applied by the governor weights at the desired maximum engine speed. The maximum speed can he controlled. Fuel is constantly bypassed to maintain the proper fuel supply pressure between idle and maximum speeds. However, such a "PT" governor does not have any means for providing a separate, i.e., independent, speed signal which may be used, for example, for controlling timing pressure as a function of speed. Furthermore, since torque shaping, via the weight assist and torque springs, is integrated into the speed governor, separate controlling of the individual functions of torque shaping and governing is not very easy. Thus, there is a need for a hydromechanical control arrangement which will enable easier control of the individual torque shaping and governing functions, while at the same time providing a speed signal that may be utilized as a engine speed parameter for control of the pressure of the timing fluid supply.